The present invention relates to a high-capacity optical information recording medium, a manufacturing method thereof, and an optical information recording and reproducing apparatus.
The progress of information-oriented society in recent years using optical communications has been requiring the construction of a communication system capable of communicating a large amount information at high speed. Optical devices essential for deploying such high-capacity high-speed optical communication include optical information recording media for storing high-volume optical information. Further, with the improvement of quality of video images (e.g. television pictures) by digitization, high definition technology, etc., the development of high-capacity optical information recording media, capable of maintaining the video images in high quality states and allowing long-duration recording, is an urgent necessity.
As the optical information recording media, DVDs having a high storage capacity of 4.7 GB per side are in practical use and widely available today not only for computers but also as media for handling high-capacity video. The DVDs are widely used not only as read-only DVD-ROMs (in which information has been directly written on a substrate) but also as rewritable recording/reproducing media. As above, development for achieving higher record density of optical information recording media is in progress, in which a laser beam of a wavelength 650 nm still shorter than 780 nm for CDs, etc. is used as a means for achieving high-density information recording. However, in order to handle high-volume information for computer graphics, digital HDTV (High-Definition TV), etc. 4-5 times the present record density has to be attained. To achieve such high record density, optical discs employing a blue laser diode (wavelength: 405 nm) are being developed and an optical disc having a storage capacity of 23.3 GB/side has been brought into practical use.
The 23.3 GB/side optical disc is capable of storing two hours of HDTV broadcasting of high definition; however, to record content exceeding two hours, recording image quality has to be reduced. Therefore, in order to realize long-duration recording/reproducing without reducing the image quality, optical discs of still higher storage capacity have to be developed. However, it is thought to be difficult to further increase the storage capacity of the current 23.3 GB optical disc for the blue laser by use of conventional methods like shortening the wavelength, increasing NA (Numerical Aperture) of the lens, etc. Since the beam for the information read/write changes into an ultraviolet beam in the stream of reduction of wavelength, components (lens, resin, etc.) of optical discs and optical disc devices have to be designed to be transparent for ultraviolet light, requiring a considerable change in materials. Further, it becomes more difficult than ever to develop laser diodes or semiconductor lasers that are inexpensive and capable of emitting ultraviolet light stably.
In increasing NA of the lens, the distance between the lens and the disc decreases and dust on the disc easily scatters light, which requires the discs to be used in a dust-free environment, impairing exchangeability as an advantage of optical discs.
As above, a new method for increasing the storage capacity of optical information recording media is becoming indispensable. As techniques for further increasing the capacity of optical discs, multi-layer recording, multilevel recording, super-resolution recording, etc. are being developed. The super-resolution recording is one of the most powerful techniques in charge of the development of next-generation high-capacity optical information recording media.
The super-resolution recording is one of high-capacity recording techniques which is achieved by a reversible change in the optical constant (refractive index (n), extinction coefficient (k)) of a super-resolution layer which is formed in the multilayer structure (recording layer, protective layer, reflecting layer, etc.) of an optical disc. In the super-resolution recording technique, a laser beam is applied to the recording surface of an optical disc while reducing the laser beam diameter by use of a laser beam condensing function or masking function of the super-resolution layer. When irradiated with the laser beam for information read/write, the irradiated part of the super-resolution layer shifts into an excited state due to an increase in temperature or absorption of photons, changing its refractive index and extinction coefficient during the irradiation. Without the laser beam irradiation, the super-resolution layer returns to its original state. Reproduction of data stored in the optical disc is carried out by irradiating the disc with the laser beam, receiving reflected light from the disc with a pickup, and discriminating between recorded parts and unrecorded parts based on the light amount of the reflected light returning to the pickup. The area of the light returning to the pickup can be made smaller than that of the laser beam irradiating the disc by use of the reversible change in the optical constant of the super-resolution layer. In other words, the resolution can be increased by reducing the size of the area of data reading, with the optical mask effect of the super-resolution layer.
Incidentally, the extinction coefficient (k) is a quantity proportional to the light absorption coefficient of the material, increasing with the increase of the absorption coefficient. The two coefficients (refractive index (n), extinction coefficient (k)) are collectively called the optical constant.
The super-resolution layer has been implemented by thin layer materials mainly composed of cobalt oxide, etc. (see JP-A-10-340482). With the super-resolution effect in consequence of the remarkable change in the refractive index of the super-resolution layer, it becomes possible to obtain optical discs of higher storage capacity.